View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by St Andrews Research Repository The Irrawaddy River Sediment Flux to the Indian Ocean: The Original Nineteenth-Century Data Revisited R. A. J. Robinson, M. I. Bird, Nay Win Oo,1 T. B. Hoey,2 Maung Maung Aye,3 D. L. Higgitt,4 Lu X. X.,4 Aung Swe,5 Tin Tun,6 and Swe Lhaing Win3 School of Geography and Geosciences, University of St. Andrews, St. Andrews, Fife KY16 9AL, United Kingdom (e-mail: [email protected]) ABSTRACT The Irrawaddy (Ayeyarwady) River of Myanmar is ranked as having the fifth-largest suspended load and the fourth- highest total dissolved load of the world’s rivers, and the combined Irrawaddy and Salween (Thanlwin) system is regarded as contributing 20% of the total flux of material from the Himalayan-Tibetan orogen. The estimates for the Irrawaddy are taken from published quotations of a nineteenth-century data set, and there are no available published data for the Myanmar reaches of the Salween. Apart from our own field studies in 2005 and 2006, no recent research documenting the sediment load of these important large rivers has been conducted, although their contribution to biogeochemical cycles and ocean geochemistry is clearly significant. We present a reanalysis of the Irrawaddy data from the original 550-page report of Gordon covering 10 yr of discharge (1869–1879) and 1 yr of sediment concentration measurements (1877–1878). We describe Gordon’s methodologies, evaluate his measurements and calculations and the adjustments he made to his data set, and present our revised interpretation of nineteenth-century discharge and sediment load with an estimate of uncertainty. The 10-yr average of annual suspended sediment load currently cited in the literature is assessed as being underestimated by 27% on the basis of our sediment rating curve of the nineteenth- century data. On the basis of our sampling of suspended load, the nineteenth-century concentrations are interpreted to be missing about 18% of their total mass, which is the proportion of sediment recovered by a 0.45-mm filter. The MT. Our revised estimate of the annual sediment load 60 ע new annual Irrawaddy suspended sediment load is364 from the Irrawaddy-Salween system for the nineteenth century (600 MT) represents more than half the present-day Ganges-Brahmaputra flux to the Indian Ocean. Since major Chinese rivers have reduced their load due to damming, the Irrawaddy is likely the third-largest contributor of sediment load in the world. Introduction The eastern syntaxis of the Himalayas and the Ti- ween, Irrawaddy, Ganges, and Brahmaputra rivers, betan Plateau contains the most tectonically com- and two of these systems, the Ganges-Brahmaputra plex geology in Asia (Socquet and Pubellier 2005). (G-B) and the Irrawaddy-Salween (I-S), debouch into This region is drained by the Red, Mekong, Sal- the Indian Ocean. We consider the Irrawaddy and Salween with the Sittoung and four smaller rivers Manuscript received January 26, 2007; accepted June 18, together because their catchments are adjacent and 2007. both flow into the Gulf of Martaban (fig. 1) along 1 Department of Geography, University of Pyay, Pyay, a similar length of coastline to the G-B river sys- Myanmar. tem. The headwaters of the I-S are undergoing some 2 Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom. of the highest rates of landscape adjustment in a 3 Department of Geography, Yangon University of Distance region of active orogenesis (Zeitler et al. 2001). Education, Yangon, Myanmar. Collision, uplift, and shear zone development 4 Department of Geography, National University of Singa- since the early Tertiary have resulted in a complex pore, Kent Ridge 117570, Singapore. pattern of river capture involving the Irrawaddy, 5 Department of Geography, University of Yangon, Yangon, Myanmar. Salween, Red, Yangtze, Mekong, and Brahmaputra 6 Department of Geography, University of Mawlamyine, rivers (Clark et al. 2004). These large rivers deliver Mawlamyine, Myanmar. material from the Himalayas to the ocean, and [The Journal of Geology, 2007, volume 115, p. 629–640] ᭧ 2007 by The University of Chicago. All rights reserved. 0022-1376/2007/11506-0002$15.00. DOI: 10.1086/521607 629 630 R. A. J. ROBINSON ET AL. close to half of the current total flux of water, sed- iment, and dissolved load from the Himalayas and Tibet to the ocean, with ∼20% of this attributed to the I-S (Milliman and Meade 1983). The Irrawaddy River ranks fifth in the world in terms of suspended load (265 MT yrϪ1) and fourth in terms of dissolved load (Milliman and Meade 1983, their table 2); these published sediment and water fluxes are de- rived from a nineteenth-century data set (Gordon 1885), and the dissolved loads were published in an article by Meybeck and Ragu (1995). Stamp (1940) states that 48 MT yrϪ1 should be added as dissolved load for the Irrawaddy but gives no source for this estimate. Values published by Meybeck and Ragu (1995), Meade (1996), Gaillardet et al. (1999), and Meybeck et al. (2003) equate to combined I-S an- nual averages of 697 km3 of water, 365 MT of sus- pended material, and 162 MT of dissolved material. Although figures for these rivers are widely quoted and used for global flux calculations of carbon (e.g., Subramanian and Ittekkot 1991), there are no mea- surements of suspended load for the Salween in Myanmar, and the values quoted by Meade (1996, his table 1) are estimates based on the measured suspended load values of the Irrawaddy and Me- kong. The increase in damming along the Yangtze (Changjiang) and Yellow (Huanghe) rivers has sub- stantially reduced the sediment load of these rivers (Walling 2006), such that the Irrawaddy may now rank as having as high as the third-largest sediment load in the world after the Amazon and the G-B. We have reanalyzed the original nineteenth- century Irrawaddy data and subsequent early twen- tieth-century engineering reports that identify problems with the data collection. In their seminal article, Milliman and Meade (1983) use the Irra- waddy as an example of the potential error that can arise when compiling global sediment budgets us- Figure 1. Location of the original Irrawaddy study site ing data recycled from previous reports rather than of Gordon (1879–1880, 1885) at Seiktha and the rivers original, often inaccessible, data. Given the very included in this article. Our measurement sites for mod- significant contribution of the Irrawaddy River to ern discharge and material fluxes are at Seiktha and Pyay global sediment budgets, we have begun to compare on the Irrawaddy and at Hpa-An on the Salween. the revised nineteenth-century water and sediment load values to our own field measurements col- knowledge of the magnitude of fluxes, as well as lected during the onset of the 2005 and 2006 mon- the mineralogical and chemical characteristics of soon seasons. We use our preliminary data set here the transported material, is a prerequisite for un- simply to compare the efficiency of Gordon’s and derstanding the impact of Himalayan uplift on our filtering methods; we do not present any com- global biogeochemical cycles (Raymo et al. 1988) parison of nineteenth-century and modern sedi- and the impact of human activities on recent-mod- ment load values because this is the focus of on- ern sediment yields (Wilkinson 2005). An under- going research. standing of denudation rates in areas of active tec- tonism is critical in assessing the extent to which Study Area and Methods climate-driven erosion influences large-scale tec- tonic behavior (An et al. 2001; Zeitler et al. 2001). The Irrawaddy and Salween rivers are ∼2000 and Together, the G-B and I-S are thought to deliver ∼2800 km long and have drainage areas of Journal of Geology IRRAWADDY SEDIMENT FLUX 631 0.413 # 1066and 0.272 # 10 km2, respectively (Nyi Goldsmith Forshey at hydrological stations on the 1967; Bender 1983). The Irrawaddy catchment rock Mississippi River between 1848 and 1855 during types include Cretaceous to mid-Cenozoic flysch the U.S. government’s Mississippi Delta Survey. of the western Indo-Burman ranges, Eocene- Gordon made daily measurements of flow velocity Miocene and Quaternary sediments of the Myan- and depth and recorded stage during 1872–1873 and mar Central Basin, and the Late Precambrian and again in 1875, missing only Sundays, holidays, and Cretaceous-Eocene metamorphic, basic, and ultra- days of very bad weather. Channel width at Seiktha basic rocks of the eastern syntaxis of the Himala- ranges from ca. 700 m at low flow to ca. 1600 m yas. The Salween and eastern tributaries of the Ir- at peak flow. Flow depth was measured along a line rawaddy drain Precambrian, Oligocene-Tertiary normal to the main flow from a boat using a lead sedimentary, and acidic and metamorphic rocks of and line, and the boat position was surveyed by the eastern Shan Plateau (Bender 1983; Curray triangulation from the bank with theodolites. Flow 2005; Najman 2005; Socquet and Pubellier 2005). velocity was estimated from the time taken for Gordon (1885) presented monthly discharge data floats to travel between two cross-channel base- collected between 1869 and 1879 and 1 yr of sed- lines spaced about 60 m apart, and theodolites po- iment load data (1877–1878) to the Royal Geo- sitioned at the end of each baseline were used to graphical Society (RGS) without details of his field track float trajectories. The starting and final po- methods or sampling locations. Gordon’s original sitions of the floats were drafted as scaled drawings, report, containing his full daily discharge, rainfall, and the downstream component of velocity was and sediment concentration data set, survey maps, resolved trigonometrically from the time of travel.